Primary productivity: gross and net

A salary and its take-home

Think of a worker who earns a salary. Some of it is spent on travel and food for the worker. What is left is the take-home pay.

A green plant is the same. It makes organic matter by photosynthesis. It also burns some of that matter in its own respiration. What is left is the part that can be eaten, or can die and be used by decomposers.

Primary production and productivity

Primary production is the amount of biomass, or organic matter, that plants make by photosynthesis in a unit area during a period of time. It is given in grams of matter for each square metre ($\text{g/m}^2$) or in kilocalories for each square metre ($\text{kcal/m}^2$).

A place can make a large amount in one year and a small amount in another. So we use the rate, which is called productivity. It is given as $\text{g/m}^2/\text{yr}$ or $\text{kcal/m}^2/\text{yr}$. A rate lets us compare ecosystems.

Gross and net

• Gross primary productivity (GPP) is the rate of production of organic matter by photosynthesis. It is the "gross salary".
• Plants use a part of the GPP in respiration. Call it $R$.
• Net primary productivity (NPP) is what is left.

The NPP is the biomass that is available to the heterotrophs, which are the herbivores and the decomposers. It is the NPP that feeds the rest of the ecosystem.

You should find that the NPP rises when the GPP rises, and falls when the plants burn a larger share. The NPP can never be more than the GPP.

A note on units

NCERT uses kilocalories. A-Level, IB and AP use joules. One kilocalorie is $4.184\ \text{kJ}$. A lesson that gives both is not contradicting itself. It is only using two units for the same energy.

The whole biosphere

NCERT gives the figures for the Earth. The annual net primary productivity of the whole biosphere is about $170$ billion tons of dry matter. The oceans cover about $70\%$ of the surface, yet they make only about $55$ billion tons of it. So the land, with $30\%$ of the surface, makes the rest, which is $115$ billion tons.

The ocean is large, but its productivity for each unit of area is small. The program below works out how much smaller.

Try it in Python

The first program follows three model places. It works out the NPP and the share of the GPP that the plants burn.

The second program uses the figures of NCERT for the whole biosphere.

Practise with new numbers

Common mistakes

• Calling the GPP the food of the herbivores. The herbivores get the NPP. The plants burn a part of the GPP first.
• Saying that productivity is an amount. Productivity is a rate. It has a time in its unit.
• Thinking that the oceans make most of the production because they are large. They make less than a third.
• Mixing kilocalories and kilojoules. One kilocalorie is $4.184\ \text{kJ}$.

Remember these

• GPP is the rate of production of organic matter by photosynthesis. $\text{NPP} = \text{GPP} - R$.
• NPP is the biomass available to the herbivores and the decomposers.
• Productivity is a rate. Its unit is $\text{g/m}^2/\text{yr}$ or $\text{kcal/m}^2/\text{yr}$.
• Annual NPP of the biosphere: about $170$ billion tons of dry matter. The oceans, with about $70\%$ of the surface, make only about $55$ billion tons.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Define gross primary productivity and net primary productivity.

2. The GPP of a field is $1800\ \text{g/m}^2/\text{yr}$ and the plants respire $750\ \text{g/m}^2/\text{yr}$. What is the NPP?

3. Who uses the NPP of an ecosystem?

4. Why is productivity given with a time in its unit?

5. How much of the NPP of the biosphere is made in the oceans? What share of the surface do they cover?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 12, Ecosystem, section on productivity.
• OpenStax Biology 2e (2018), chapter 46, Ecosystems, section on energy flow through ecosystems.
• Campbell Biology, 12th edition (2021), Pearson, chapter 55, Ecosystems and Energy, section on primary production.